drm/i915: Tune down MCH_SSKPD values warning

Users often can't do anything about this since their vendors stopped
providing BIOS updates. Also we seem to be able to hack around it
with increased latency values, and thus far the only reports have
been for screens with really high resolutions. So tune it down to a
level where only developers can see it.

Also drop some of the end-user fluff.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>